WASHINGTON -- President Bush outlined a bold new agenda for exploring space Wednesday, pledging a return to the moon as early as 2015, then pushing to Mars and beyond.

"It is time for America to take the next steps," he said during a speech at NASA headquarters.

The election-year plan, announced nearly one year after the loss of space shuttle Columbia and its seven astronauts, is the most ambitious since President Kennedy promised the first moon landing more than four decades ago.

Bush said he would add $1 billion to NASA's $15.5 billion budget over the next five years and reallocate $11 billion in existing space agency programs to support the moon-Mars effort.

But skeptics of the plan said the president's goals likely would cost hundreds of billions of dollars -- money that might be scarce as the federal deficit is $500 billion a year and rising.

When Bush's father proposed a similar Mars mission in 1989, the plan collapsed on Capitol Hill when lawmakers rejected the huge price tag.

But on Wednesday, an upbeat Bush sold the plan as a way to give NASA a new focus.

"We will build new ships to carry man forward into the universe, to gain a new foothold on the moon, and to prepare for new journeys to worlds beyond our own," he said.

Comparing space exploration to the efforts of Lewis and Clark, who charted the nation's western reaches after the Louisiana Purchase, Bush said the desire to explore and understand "is part of our character."

More details of Bush's plan will come when he submits NASA's proposed budget to Congress on Feb. 2. On Wednesday, he called for robotic missions to the moon by 2008, the development of a new "crew exploration vehicle" by 2014 and a human mission to the moon sometime between 2015 and 2020.

The president also wants to return the nation's three shuttles to flight -- possibly as early as this fall -- and to complete construction on the International Space Station. After that, NASA would retire the shuttles and the United States would turn over much of the station's operation to foreign partners.

Mars exploration would come later, after NASA develops the technology and spacecraft to carry humans to the Red Planet and after it determines how to keep astronauts healthy in space to complete a years-long round trip.

Bush also assigned former Air Force Secretary Pete Aldridge, a Houstonian, to head a presidential advisory commission to decide how to accomplish the goals. The commission's first report is due within four months of its first meeting.

Bush said he envisions the moon as a staging ground for assembling and launching new spacecraft for missions to "extend a human presence across our solar system."

The announcement was the second big initiative Bush has unveiled in two weeks. The first was a plan to overhaul the nation's immigration laws to allow foreigners to work in the United States legally by issuing renewable three-year guest worker permits.

Bush said NASA has been stuck in near-Earth orbit for too long.

Robotic explorers, such as the Spirit rover that recently landed on Mars and another that is en route, would be "trailblazers," Bush said. But he added that humans must follow.

"In the past 30 years, no human being has set foot on another world or ventured farther up into space than 386 miles, roughly the distance from Washington, D.C., to Boston, Mass.," he said.

"The human thirst for knowledge ultimately cannot be satisfied by even the most vivid pictures or the most detailed measurements," Bush added. "We need to see and examine and touch for ourselves."

Lawmakers who attended Bush's speech applauded the president for settling on a bold vision for the space agency, which has struggled since the Columbia disaster.

Before delivering his speech to a packed auditorium at NASA headquarters, Bush was welcomed by NASA Administrator Sean O'Keefe and, via video, by astronaut Michael Foale aboard the space station.

"I know that I'm just one chapter in an ongoing story of discovery," Foale told the president.

Bush noted that Foale is working on the space station with Russian cosmonaut Alexander Kaleri, and said that like the space station project, his new exploration plans would welcome foreign participation.

"The vision I outline today is a journey, not a race," Bush said, "and I call on other nations to join us on this journey, in a spirit of cooperation and friendship."

The president said mankind is drawn to space for the "same reason we were once drawn to unknown lands and across the open sea."

"We choose to explore space because doing so improves our lives and lifts our national spirit," he said. "So let us continue the journey."